Methods of reproducing digital binary images (also termed "half-tone images") are generally known. A charged photo-conductive medium is exposed pixel-wise and thus discharged by way of a light source, e.g., an array of LEDs which is disposed transversely to the direction of transport of the photo-conductive medium. The result is a charge image formed of charged and discharged areas. The photo-conductive medium is then passed along a developing unit, usually a rotating magnetic roller with a developing powder. In these conditions the charge image is developed with toner.
There are two alternative systems with regard to exposure. In a first system, charged areas on the photo-conductive medium are developed while the discharged areas remain free. A system of this kind is termed a "white-writer". In the other system the converse is the case. This is achieved by applying, between the developing unit and the photo-conductive medium, an electrical voltage difference equal to the surface potential of charged areas of the photo- conductive medium. A system of this kind is termed a "black-writer".
In electrophotographic printers, the light source is usually such that it exposes a somewhat larger area than corresponds to the dimensions of a pixel. The reason for this is to prevent unexposed areas from remaining between the pixels. In the case of white- writers the result of this is that the areas developed with toner powder become smaller than intended, and this becomes particularly visible as a narrowing of lines and partial or complete disappearance of fine details. In black-writers, on the other hand, the areas developed with toner powder become somewhat larger, so that narrow lines are conspicuously widened and, for example, small letter characters merge.
Another effect that influences the edges of developed image portions is that in many developing systems, particularly those using developing powders of relatively high resistivity, electrical edge fields occur which cause enhanced development of toner at and just outside the edge of areas for development. In the case of white-writers, this electrical edge effect counteracts the said exposure effect and partly cancels it out, but in black-writing systems the two effects work in the same direction, so that action is required particularly in the case of the latter systems.
An obvious remedy to counteract extension of the exposed area would be to make the area exposed per pixel smaller. This can be done, for example, by reducing the intensity of the light source. The intensity of the light spot projected on to the photo- conductive medium for a pixel decreases from the center, frequently approximately like a Gaussian curve. A reduction of the intensity is therefore first perceptible at the edges of the light spot because a larger proportion of the descending flanks falls below the sensitivity threshold of the photo-conductive medium.
A disadvantage of this remedy, however, is that the discharge is locally no longer complete in larger exposed areas. In black-writers, the remaining residues of the charge obstruct the development so that the optical density of a print becomes unacceptably low in larger image areas. In white-writers, the remaining charge residues cause a (slight) development of toner powder in areas which should have remained white.
A second remedy to counteract the described undesired extension of the exposed area comprises shifting the time when the light source is switched on when the latter enters and leaves respectively an area for exposure during its relative movement with respect to the photo-conductive medium, i.e., variable timing for the light source. In the case of a white-writer, a method of this kind is described in U.S. Pat. No. 4,387,983. In this way the edge of an exposed and hence discharged area can be so shifted on the photo-conductive medium that the boundary of the developed area is situated exactly at the required location. A disadvantage of this solution is that it requires intervention in the timing of the light source control. Since high-frequency signals are involved here, specialized expensive components must be used for implementation. Moreover, in the case of LED arrays particularly, the control circuits associated with the commercially available products are often not suitable for carrying out small timing changes applicable solely to portions of the array. LED arrays in which this is possible are very expensive.